וַיֹּאמֶר לֶמֶךְ לְנָשָׁיו:
"עָדָה וְצִלָּה שְׁמַעַן קוֹלִי, נְשֵׁי לֶמֶךְ הַאְזֵנָּה אִמְרָתִי
כִּי אִישׁ הָרַגְתִּי לְפִצְעִי, וְיֶלֶד לְחַבֻּרָתִי
כִּי שִׁבְעָתַיִם יֻקַּם קָיִן, וְלֶמֶךְ שִׁבְעִים וְשִׁבְעָה."
What does this poem mean? What does Lemech intend to say, why does he intend to say it, and why is it included in the Torah?
Let us begin analyzing the poem by breaking it down into three lines:
1 - The introduction, calling on Lemech's wives
2 - The line about killing a man and child
3 - The line about vengeance
The most ambiguous line of the three is #2, and I purposefully did not punctuate it above so as to preserve the ambiguity. Essentially, there are two ways of reading it. Either it is a statement:
"For I have killed a man over my injury, and a child over my wound."
Or else, it is a rhetorical question from someone who has not killed:
"Have I killed a man over my injury, or a child over my wound?"
I think the best way to approach this ambiguity is by asking of Lemech was a good or bad person. If he was a good person, then most likely the question is rhetorical, and Lemech has not killed (though some commentators suggest that he had killed unintentionally and wanted to protest his innocence of murder). In the "good Lemech" case, the vengeance in #3 must be God's vengeance against whoever would harm Lemech.
The alternative is to say that Lemech was a bad (or at least violent) person. He is stating for a fact that he had killed. In that case, the vengeance is carried out by Lemech himself against whoever would attempt to harm Lemech.
My difficulty with the "good Lemech" interpretation is that nowhere do we hear that God has promised vengeance upon those who attack Lemech. You would think that this promise should be mentioned in the Torah, so that we know what Lemech is talking about. Since no such story exists, I prefer the "bad Lemech" interpretation in which Lemech promise that he himself will carry out the vengeance.
In that case, line #2 should be translated as follows: "For I have killed a man over my injury, and a child over my wound." This killing is disproportionate: Lemech received a non-fatal wound, and in return he inflicts a fatal injury. That's not very moral, but it does fit perfectly the claim in line #3 that Lemech will take 77-fold vengeance. R' Yaakov Medan, in his book "Ki Karov Elecha", goes further and explains the likely details of Lemech's killing: he killed a "man" and a "child", a man because that man wounded him, and a child (the same man's child) because the most total revenge against someone is to kill them and all their offspring!
So now we have a coherent explanation of everything in lines #2 and #3. What about #1? Why does this story have to be told to Lemech's wives?
Perhaps we can draw an answer from the fact that there are TWO wives. Having multiple wives is not a recipe for happiness in the Torah. In halacha, each wife is known as the "tzara" of the other, due to the hostility they are presumed to have for each other as they vie for their husband's affection. And not only do they fight with each other, but their struggles are likely to spill over and disturb the husband's life. Avraham was forced to part forever from his son Yishmael due to his wives' rivalry, David and Shlomo each suffered incredibly due to their later wives (Batsheva and Shlomo's foreign wives), and even someone as highly regarded as Yaakov Avinu was brought to rage by one of his wives' complaints (Breishit 30:2). We already know Lemech was a over-reacting violent man, and with the extra stress caused by his bigamy, we can imagine this was not a peaceful household.
Perhaps, given this situation, we can explain why Lemech chose to give this speech to his wives. Perhaps there had been one fight too many in the household, and Lemech had had enough. He knew only one way of solving conflicts, and that was to utterly destroy the other side. Thus, in this poem, he threatened that if one of his wives went a bit too far in the future, he would kill her. Given Lemech's past, this would not be an idle threat. And since Lemech had TWO wives, killing one of them might not be an overwhelming blow to him, as he would still have the other. If this idea is correct, then here we have the first and perhaps the only mention in the Torah of marital violence.
Lemech's poem is the last word in the story of Kayin's descendants: the next verse (4:25) tells of the birth of Shet, who would eventually become the ancestor of Noach. In some ways Lemech's life could be seen as a success: his three sons go on to found the disciplines of herding, music, and forging (4:20-22). As with Joseph Kennedy, whose children included a president and two senators, one has to suspect that such accomplished kids owe something to the parents. But Lemech was a violent, vindicative man - not only to strangers but, it seems likely, even to his own wives. That quality is the "death sentence" for Lemech. Once the Torah attests to his negative character traits, it immediately and permanently abandons him, turning its attention to the family which will in part survive the flood.